Scatter the onions on the bottom of a slow cooker. Place chicken on top of the onions.

In a medium bowl, combine the ketchup, vinegar, mustard, molasses, onion powder, cumin, garlic powder, Tabasco and salt. Pour the sauce over the chicken. Cover the slow cooker and cook on low until the chicken is fork-tender and the sauce has thickened slightly, about 5-6 hours (add an additional hour if using frozen chicken breasts).

Remove the chicken from the slow cooker and shred the meat. Return it back to the slow cooker, mixing it with the sauce. Serve the chicken hot on whole wheat rolls, if desired.